<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .page {
            padding: 0 15px;
        }
        
        .header,
        .nav,
        .main,
        .footer {
            background-color: green;
            height: 100px;
            text-align: center;
            border-radius: 50px;
            color: white;
            line-height: 100px;
            /* margin: 0 30px; */
            /* margin-bottom: 10px; */
        }
        /* .page>div:nth-child(4) {
            margin-bottom: 0;
        } */
        /* .page>div:last-child {
            margin-bottom: 0;
        } */
        /* .page>div+div:last-child {
            margin-bottom: 0;
        } */
        
        .page>div:not([class="footer"]) {
            margin-bottom: 10px;
        }
        
        .main {
            display: flex;
        }
        
        .main>.article {
            flex: 1;
            background-color: red;
            margin-right: 30px;
        }
        
        .main>.aside {
            width: 300px;
            /* flex: 1; */
            background-color: pink;
        }
    </style>
</head>

<body>
    <div class="page">
        <div class="header">header</div>
        <div class="nav">nav</div>
        <div class="main">
            <div class="article">article</div>
            <div class="aside">aside</div>
        </div>
        <div class="footer">footer</div>
    </div>

</body>

</html>